我对数据库关系代数中减法sql实现的思考：mysql脚本

一、创建数据库，创建表结构

CREATE DATABASE Test_sub DEFAULT CHARACTER SET utf8;

USE Test_sub;

CREATE TABLE student(
	id INT PRIMARY KEY AUTO_INCREMENT, -- 学号 id
	NAME VARCHAR(10) -- 姓名 name
);

CREATE TABLE course(
	id INT PRIMARY KEY  AUTO_INCREMENT,  -- 课程号 id
	NAME VARCHAR(10) -- 课程名 name
);

CREATE TABLE sc(
	student_id INT,
	course_id INT,
	PRIMARY KEY(student_id, course_id),
	CONSTRAINT student_fk FOREIGN KEY(student_id) REFERENCES student(id) ON UPDATE CASCADE ON DELETE CASCADE,
	CONSTRAINT course_fk FOREIGN KEY(course_id) REFERENCES course(id) ON UPDATE CASCADE ON DELETE CASCADE	
);

二、插入数据

INSERT INTO student(NAME) VALUES
('aaa'), ('bbb'), ('ccc'), ('ddd');

INSERT INTO course(NAME) VALUES
('课程_1'), ('课程_2'), ('课程_3'), ('课程_4');

INSERT INTO sc VALUES
(1, 1), (1, 2), (1, 3), (1, 4), 
(2, 1), (2, 2), (2, 3), 
(3, 1), (3, 2), 
(4, 1);

三、操作

查询学生ddd未选的课程：使用左外连接，并查询为空的

SELECT course.name
-- select * 
FROM
course LEFT JOIN 
(
	SELECT *
	FROM sc
	WHERE sc.student_id = 
	(
		SELECT id
		FROM student
		WHERE NAME = 'ddd'
		-- limit 0,1
	)
) AS sc_1 
ON course.id = sc_1.course_id
WHERE sc_1.student_id IS NULL;

补：课本案例，实现左外、右外：

1.创建表结构：

CREATE TABLE R(
	A CHAR(2),
	B CHAR(2),
	C CHAR(2)
);

CREATE TABLE S(
	B CHAR(2),
	E CHAR(2)
);

2.插入值：

INSERT INTO R VALUES
('a1', 'b1', '5'),
('a1', 'b2', '6'),
('a2', 'b3', '8'),
('a2', 'b4', '12');

INSERT INTO S VALUES
('b1', '3'),
('b2', '7'),
('b3', '10'),
('b3', '2'),
('b5', '2');

3.测试左外：

SELECT * 
FROM  r 
LEFT JOIN s  ON r.b = s.b;

4.测试右外：

SELECT * 
FROM r
RIGHT JOIN s ON r.b = s.b;